In conventional Light Emitting Diodes (LEDs), a phosphor containing material is placed directly on top of the p-n junction of the LED so that both heat sources are directly adjacent to each other. Heat may be conducted through a sapphire substrate to the mounting material of the LED chip, but because sapphire is a poor thermal conductor compared to metallic materials, this is not very efficient.
In the field of LEDs, there are at least two general classes of devices with small area, high luminous flux light emitting surfaces (LESs). They are Chip Scale Packages (CSPs) and Chip on Board (COB) devices. CSPs may be defined by a ratio between the area of the LED chip and the total area of the package. COB devices incorporate a relatively large number of closely spaced LED chips in one or more arrays that are mounted on a metal core circuit board.
In contrast to conventional LEDs, the present inventors recognized that in a CSP, the p-n junction of the CSP LED chip is located adjacent to the circuit board the LEDs are mounted on. This reduces the thermal resistance between the p-n junction and the package. In addition, the phosphor materials are located on the opposite side of the sapphire substrate so the amount of heat that is conducted from the p-n junction of the LED to the phosphor materials is greatly reduced. This combination of factors greatly reduces the junction operating temperature of CSP LEDs compared to other LED devices.
One type of CSP LED comprises a so-called flip-chip LED with a sapphire substrate. The term flip-chip refers to the chip being mounted with the sapphire substrate side facing up. In this orientation, the positive and negative electrodes of the LED are located on the base or bottom surface of the LED chip when it is mounted. Electrical contact is made using thermosonic bonding or a type of solder attach. The solder used to make electrical connections may be a traditional solder material or a eutectic solder.
White CSP LEDs incorporate a phosphor film. A portion of the light emitted by the LED chip is absorbed by the phosphor material(s) and re-emitted at different wavelengths. The combination of light emitted from the LED chip and the light emitted by the phosphor is perceived as white. The ratio of the light emitted by the LED chip to the light emitted by the phosphor as well as composition of the phosphor determines the spectral power distribution of the LED device. In turn the spectral power distribution determines the chromaticity coordinates of the LED, the Correlated Color Temperature (CCT) and the color rendering properties.
Prior CSP LEDs had one of three different configurations. These configurations are shown in FIGS. 1 through 3. In each configuration, the bottom surface of the LED flip-chip 10, 20, 30 is exposed to allow solder connections. In the first configuration LED flip-chip 10 of FIG. 1, the phosphor material 18 covers the top and sidewalls of the LED chip. In the second configuration LED flip-chip 20 of FIG. 2, phosphor material 19 covers the top of the LED and white reflective materials 21 cover the sides of the LED flip chip. In the third configuration LED flip-chip 30 of FIG. 3, phosphor material 22 covers the top and sidewalls of the LED, however a transparent polymeric material 24 covers the phosphor materials so that no phosphor materials are directly exposed. The present inventors recognized that covering the sides of the LED flip-chip makes the packages larger and also requires additional processing steps. The present inventors recognized the need for a LED flip-chip that has a smaller footprint and allows a more simplified manufacturing process.
Each of the LEDs of FIGS. 1 through 3 have a sapphire substrate 17. Below the substrate 17 is one or more n-type layers 11. A buffer layer may be provided between the substrate 17 and the n-type layers 11. On the n-type layers 11 is the active region 12. The active region 12 is a series of thin layers with varying composition. The varying compositions of the active region 12 may comprise alternating layers of GaN and Indium Gallium Nitride (InGaN). After the active region 12 is provided, a series of p-type layers 13 are provided.
After deposition of the entire LED structure, the substrate 17 with the various parts of the LED structure are then processed to form a completed LED die. Since the sapphire substrate 17 is electrically insulating, portions of the p-type layer 13 and the active region 12 as well as a portion of the n-type layers 11 are etched away to expose a surface of the n-type layers 11. Electrical contacts 14, 15 are formed on top of the p-type layer and n-type layer and the device is separated to form a completed LED die.
The present inventors recognized the need for a flip chip LED that provides a different spectral power distribution emitted from the top of the flip chip LED than from the side of the flip chip LED.
To make one type of traditional COB device, a dam structure is formed around the area where the LED chips are bonded. The dam may be formed before or after the LED chips are mounted. The region inside the dam is then filled with phosphor/silicone mixture that is then allowed to cure. The present inventors recognized that the chromaticity coordinates and hence CCT and color rendering properties of traditional COB devices are fixed during production and cannot be tuned or adjusted. The present inventors recognized the need for a COB device where the chromaticity coordinates, the CCT, and/or color rendering can be adjusted after production.
One prior art COB device 40 utilizes a single type of LED die 54 mounted on a metallic support 42, as shown in FIG. 4. Dielectric layers (not shown) are coated on a major surface of the metallic support to provide electrical isolation. Electrically conductive traces 52 are formed on the dielectric layer to form a partial electrical circuit. These electrically conductive traces 52 are connected to a power source via external electrical contacts 44 to allow the LED dies 54 to be powered. LED dies 54 are mounted on bonding pads 45 and connected to the electrically conductive traces 52 to form a series/parallel array of LED devices. The LED bonding pads 45 are disposed inside a raised polymeric material called a dam 46. The dam 46 defines the LED bonding area 49 along with the LED bonding pads 45. After the LED dies 54 have been placed and connected to the electrically conductive traces 52, the interior of the dam is filled with a transparent thermosetting resin encapsulating material (not shown). White COB devices may be produced using blue light emitting GaN based LEDs where one or more phosphor materials are dispersed in the thermosetting encapsulating resin disposed inside the dam 46. The present inventors recognized that COB LED devices with a single pair of external electrical contacts 44 are not capable of altering their emitted spectral power distribution.
The COB device 40 uses individual LED dies and least one wire bond 56. The present inventors have recognized that these wire bonds 56 are fragile and may be susceptible to damage during assembly or installation of the COB LED device in a fixture. The present inventors have recognized that it would desirable to eliminate the use of fragile wire bonds.
COB LED devices 60 have been developed that include two pairs of external electrical contacts 62, 61, as shown in FIG. 5. In these COB devices, the LED die corresponding to each set of external electrical contacts 62, 61 are arranged in alternating rows. In devices with two pairs of external electrical contacts 62, 61, the first set of LEDs inside the dam 64 are encapsulated with a first phosphor material 66 while the second set of LEDs inside the dam 64 are encapsulated with a second phosphor material 68. The first set of LEDs and their encapsulating first phosphor material 66 and the second set of LEDs and their encapsulating second phosphor material 68 are thus formed in alternative lines.
When an electrical current is passed through the first set of LEDs encapsulated with a first phosphor material 66 a first spectral power distribution of light is emitted. When an electrical current is passed through the second set of LEDs encapsulated with a second phosphor material 68 a second spectral power distribution of light is emitted. By adjusting the electrical current through the first set of LEDs and the second set of LEDs, the color tunable COB may emit a net spectral power distribution that is a mixture of the first spectral power distribution and the second spectral power distribution.
The present inventors recognized that these prior art color tunable COB LEDs exhibit significant variations in perceived color over angle. The viewer will notice that the color emitted from the first phosphor material 66 is different from the color emitted from the second phosphor material 68. Therefore, the COB device will have a striped appearance from the differing emissions from the alternating rows. The present inventors recognized that this COB device suffers from poor color mixing, especially in the near field. The present inventors recognized it would be desirable to provide a color tunable COB device with increased apparent color uniformity between different sets of LEDs contained in the COB.
The present inventors further recognized that there is a relationship between efficiency or efficacy of LED and the phosphor materials used to produce a certain CCT and color rendering property. This in turn results in non-uniform emission intensity across the LES that correlates with the color mixing issues. The present inventors recognized the need for a COB device with post-manufacturing color mixing capabilities as well as improved color mixing capabilities.
The present inventors recognized that certain CSP LEDs are relatively fragile and easy to damage. This complicates handling and mounting the CSP based COB devices. Great caution must be used during assembly to prevent damage. This sensitivity to physical damage may also necessitate design changes that increase complexity and production cost. The gaps between the CSP packages also act as dirt and dust traps. This makes CSP based COB devices much more sensitive to dirt depreciation, e.g. loss of light due to dirt accumulation on the device. The present inventors recognized the need for a device where the CSP LEDs are better protected against damage and dirt depreciation.
The present inventors has recognized that prior arrangements for powering LEDs in COB devices either lack the ability to provide different current to different set of LEDs or comprise additional cost and complexity. One type of static color mixing of LEDs in COB devices has a first set of LEDs 71 connected in parallel with a second set of LEDs 72. Each LEDs may be connected in series within a set of LEDs 71, 72. In the configuration of FIG. 6, both sets of LEDs 71, 72 are powered by a single current source 73. Increasing or decreasing the current delivered to the LED array by the single current source 73 results in increasing or decreasing amount of light being emitted by each set of LEDs 71, 72 with no significant shifts in perceived color or relative spectral power distribution. The present inventors recognized that it would be desirable to independently control the power delivered to different sets of LEDs.
One type of dynamic color mixing of LEDs uses a first set of LED 81 connected to a first current source 84 and a second set of LED 82 connected to a second current source 83. The first current source is independent of the second current source. Each LED may be connected in series within a set of LEDs 81, 82, as shown in FIG. 7. In order to allow color tuning, the first string of LEDs 81 must emit a different spectral power distribution than the second string of LEDs 82. By changing the current delivered to the first set of LEDs 81 by the first current source 84 and the current delivered to the second set of LEDs by the second current source 83, the perceived color of the resulting spectral power distribution can be formed as some mixture of the two spectral power distributions. The present inventors recognized that using independent current sources for each set of LEDs adds cost and complexity to the resulting light source. The present inventors recognized the need for independent control of power delivered to different sets of LEDs while avoiding the cost and complexity of using independent current sources for each set.